In optical communications networks, optoelectronic modules are used to transmit and/or receive optical signals over optical fibers. The optoelectronic module may be configured as an optical transmitter that transmits optical signals, an optical receiver that receives optical signals, or an optical transceiver that transmits and receives optical signals. On the transmit side of an optical transmitter or transceiver module, a light source (e.g., a laser diode) generates amplitude modulated optical signals that represent data, which are optically coupled by an optics system of the module into an end of a transmit optical fiber. The signals are then transmitted over the transmit fiber to a receiver node of the network. On the receive side of an optical receiver or transceiver module, an optics system of the module receives optical signals output from an end of a receive optical fiber and focuses the optical signals onto an optical detector (e.g., a photodiode), which converts the optical energy into electrical energy.
In some laser-based optoelectronic modules, a portion of the light that is produced by the laser is used to monitor the optical output power level of the laser and to adjust the optical output power level of the laser as needed. Typically, it is desirable to maintain the optical output power level of the laser at a substantially constant, predetermined level during operations. To accomplish this, many optoelectronic modules include components that together make up a feedback control system for monitoring the average optical output power level of the laser and adjusting the bias and/or modulation currents of the laser as needed to maintain that average optical output power level at a substantially constant, predetermined level. The feedback path components typically include a beam splitter, a monitor photodiode, analog-to-digital (ADC) circuitry, and controller circuitry. The beam splitter causes a portion of the beam that is produced by the laser to be split off and directed onto a monitor photodiode. The monitor photodiode produces an analog electrical signal in response to the light that is directed onto it by the beam splitter. The analog electrical signal is converted into a digital electrical signal by the ADC circuitry. The controller circuitry processes the digital electrical signal and causes the laser modulation and/or bias currents to be adjusted accordingly.
Beam splitters are manufactured in a variety of configurations and typically comprise one or more reflective, refractive and/or diffractive elements. In a typical beam splitter configuration, a first portion of the main beam produced by the laser passes through the beam splitter with very little if any of the light being reflected, refracted or diffracted. This portion of the main beam is then coupled into the end of the transmit optical fiber for transmission over the transmit optical fiber. At the same time, a second portion of the main beam is reflected, refracted and/or diffracted by the beam splitter to cause the second portion to be directed onto the monitor photodiode.
Usually, the first and second portions each contain about 50% of the optical power that was contained in the main beam. This symmetric, or even, split of the optical power can cause problems in some cases. Typical lasers that are used in optoelectronic modules produce light having optical power levels that are much greater than safety limits for the human eye. Even at 50% of the optical power of the main beam, the first portion of the light will have an optical power level that is greater than eye safety limits. It is generally not possible, or at least very difficult, to run a laser at the high speed required for the optical communications link and simultaneously reduce the optical output power level of the laser to a level that is within eye safety limits. For this reason, steps are often taken to ensure that the light that is to be transmitted over the transmit optical fiber is attenuated to an optical power level that is within the safety limits.
Accordingly, a need exists for an optical beam splitter for use in an optoelectronic module that is capable of providing an uneven, or asymmetrical, split of the main beam produced by the laser such that the portion of the light that is split off and coupled into the end of the transmit optical fiber as the optical data signal has an optical power level that is within human eye safety limits and yet that has sufficient optical power to avoid signal degradation problems.